This invention relates to a lighting system comprising at least one high-pressure gas discharge lamp and at least one incandescent body or filament, and an impedance limiting the current of the gas discharge lamp wherein the gas discharge lamp and the current limiting impedance are connected in series and having the incandescent body or filament connected with them in parallel in the form of a standby filament or an incandescent lamp, wherein under the term "standby filament" one should understand an incandescent coil, arranged in a common evacuated or gas filled interior of a light-transmitting outer glass envelope, and wherein the system includes a controllable current-interrupter element connected in series with the incandescent body, and a control circuitry for the interrupter element, furthermore, the system includes a sensing element for sensing the state of the gas discharge lamp.
Efforts made nowadays to save energy have brought into the forefront the replacement of incandescent lamps of relatively poor efficiency by light sources with better luminous efficiency, particularly by low-pressure and high-pressure gas discharge lamps. However, the substitution of gas discharge lamps raises many problems. The geometrical dimensions of the low-pressure gas discharge lamps are large, while high-pressure gas discharge lamps cannot instantaneously emit light upon being switched on or upon being re-started immediately after being switched off while still warm.
It is known that steady efforts are being made to develop new light sources which combine the favorable properties of various light sources such as incandescent and high-pressure gas discharge lamps.
There have been many propositions for the operating of an incandescent lamp or "standby filament" parallel with a gas discharge lamp during the time interval when the gas discharge lamp still does not emit light, or its emitted light still does not have enough strength. FIG. 5 which will be discussed hereinafter, illustrates the time-voltage relationship within a gas discharge lamp. The initial stage of the diagram relates to the turning-on of the discharge lamp for its cold state, and later, it illustrates the voltage-time relationship during a turning-off, or repeated turning-on of the lamp in its warm state. Those time intervals are indicated where it is necessary to operate the incandescent body, because the discharge lamp still does not give off sufficient light. A large number of solutions became known to solve the problem and, wherein in addition to the gas discharge lamp, a single incandescent element is operated , which both in the turning-on in the warm state or, at the initial stage, will supplement the light of the discharge lamp. Such solutions are illustrated by the subject matter of U.S. Pat. Nos. 4,232,252 and 4,151,445. In such prior art literature, the sensing means which senses when it becomes necessary to operate the incandescent body, comprises a transformer. A transformer, as a sensing means, has a disadvantage in that it has large dimensions and it is expensive and, due to its dimensions, it cannot be placed within the lamp socket or into the neck portion, when it comes to the so-called "energy saver" lamps. Consequently, such solutions cannot be used in connection with energy saver lamps. U.S. Pat. No. 4,278,916 describes a relatively more complicated solution, according to which a separate incandescent element is used for the turning-on from the cold state and for the turning on from the warm state. In the latter solution the sensing means is represented by a resistor 28 serially connected with the discharge lamp and, as such, it will use the lamp current to a considerable extent, which will reduce the efficiency thereof, therefore, such solution again cannot be considered in connection with energy saving lamps.